1. Introduction
The occurrence of emerging pollutants (EPs)—including synthetic antibiotics, personal-care chemicals, endocrine disruptors, and agricultural pesticides—in surface waters and municipal effluents has become a global environmental crisis.
Unlike traditional legacy contaminants, EPs persist at trace concentrations (ng/L to μg/L) and resist conventional biological and physicochemical wastewater treatment. Photocatalytic Advanced Oxidation Processes (AOPs) offer a promising, sustainable pathway by harnessing solar photons to generate non-selective reactive oxygen species (ROS) capable of mineralizing organic recalcitrants into non-toxic CO₂ and H₂O.
Metal–organic frameworks (MOFs), constructed from metal nodes linked by organic ligands, feature immense surface areas (>2000 m²/g) and uniform pore architectures. Coupling visible-light active Fe-based MOFs, such as MIL-101(Fe), with robust semiconductor nanoparticles (TiO₂) forms heterojunction nanocomposites that dramatically suppress charge recombination and enhance photocatalytic degradation.
2. Materials and Experimental Methodology
Solvothermal Synthesis of MIL-101(Fe): Iron(III) chloride hexahydrate (FeCl₃·6H₂O) and 1,4-benzenedicarboxylic acid (H₂BDC) were dissolved in N,N-dimethylformamide (DMF) with controlled acetic acid modulator. The mixture was reacted at 150 °C for 15 hours in a Teflon-lined stainless steel autoclave, followed by hot ethanol washing to evacuate residual guest molecules.
Nanocomposite Assembly: Predetermined mass fractions of Degussa P25 TiO₂ nanoparticles (10, 20, and 30 wt%) were dispersed into ethanolic MIL-101(Fe) suspensions via ultrasonication, followed by slow solvothermal crystallization to ensure intimate interfacial bonding.
Physicochemical Characterization Techniques: The synthesized photocatalysts were characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and nitrogen physisorption:
| Analytical Technique | Target Property | Primary Analytical Insight |
|---|---|---|
| PXRD / XRD | Crystallographic phase integrity | Verification of MIL-101(Fe) octahedron cage reflections and TiO₂ anatase/rutile phases |
| SEM & EDX | Particle morphology & elemental mapping | Homogeneous distribution of spherical TiO₂ nanoparticles across octahedral MOF facets |
| N₂ Adsorption-Desorption (BET) | Specific surface area & pore volume | Quantification of micro- and mesoporous network retention post-composite assembly |
| UV-Vis DRS | Optical bandgap & light absorption | Extension of optical absorption edge into the visible-light spectrum (λ > 420 nm) |
3. Results and Discussion: Material Properties
Nitrogen sorption isotherms confirmed the porous hierarchy of the synthesized materials. Table 2 summarizes the textural and structural parameters:
| Photocatalyst Sample | BET Surface Area (m² g⁻¹) | Total Pore Volume (cm³ g⁻¹) | Average Particle Size (nm) |
|---|---|---|---|
| Pristine MIL-101(Fe) | 2,210 | 1.32 | 85 |
| Commercial TiO₂ (P25) | 78 | 0.18 | 32 |
| MIL-101(Fe)/TiO₂ (10 wt%) | 860 | 0.82 | 68 |
| MIL-101(Fe)/TiO₂ (20 wt%) | 412 | 0.56 | 58 |
| MIL-101(Fe)/TiO₂ (30 wt%) | 295 | 0.41 | 52 |
4. Photocatalytic Degradation and Kinetic Analysis
Photocatalytic evaluation was performed using ciprofloxacin (CIP, C₀ = 10.0 mg/L) under solar simulation. Pristine TiO₂ achieved only 42% degradation after 120 minutes due to poor visible absorption, while pure MIL-101(Fe) reached 54%.
The 20 wt% MIL-101(Fe)/TiO₂ nanocomposite demonstrated outstanding synergy, reaching 95.3% CIP degradation (residual concentration 0.47 mg/L). Reaction kinetics fit pseudo-first-order behavior: ln(C₀/C) = k_app · t.
| Catalyst Formulation | Apparent Rate Constant k_app (min⁻¹) | Half-Life t₁/₂ (min) | CIP Removal at 120 min (%) |
|---|---|---|---|
| Blank (Photolysis only) | 0.002 | 346.5 | 7.2% |
| Pristine TiO₂ | 0.010 | 69.3 | 42.1% |
| Pure MIL-101(Fe) | 0.014 | 49.5 | 54.0% |
| MIL-101(Fe)/TiO₂ (10 wt%) | 0.017 | 40.8 | 82.4% |
| MIL-101(Fe)/TiO₂ (20 wt%) | 0.022 | 31.5 | 95.3% |
| MIL-101(Fe)/TiO₂ (30 wt%) | 0.019 | 36.5 | 88.6% |
5. Operational Parameters, Reusability, and Quality Control
Reusability: Over 5 consecutive cycles, the 20 wt% composite maintained >91% degradation efficiency. ICP-OES analysis revealed negligible iron and titanium leaching (<0.08 mg/L), confirming excellent structural stability.
Radical Scavenging: Quenching tests using benzoquinone (BQ), isopropanol (IPA), and EDTA confirmed that superoxide radicals (•O₂⁻) and photogenerated holes (h⁺) are the dominant active species in mineralization.
Quality Control and Error Mitigation: Rigorous experimental protocols were instituted to control potential experimental artifacts:
| Source of Error | Potential Experimental Effect | Mitigation / Control Strategy |
|---|---|---|
| Irradiance Variation | Fluctuation in reaction kinetics | Continuous monitoring with calibrated radiometer at fixed 10 cm lamp distance |
| Sampling Delay | Inaccurate post-sampling reaction | Immediate microfiltration (0.22 μm PTFE) and prompt HPLC refrigerated autosampling |
| Photocatalyst Settling | Mass-transfer resistance | Constant magnetic stirring at 500 rpm in jacketed photoreactor |
| pH Drift | Alters surface charge and ROS chemistry | Initial and final pH verification with digital buffer-calibrated probe |
6. Conclusion and Future Directions
This study demonstrates that MIL-101(Fe)/TiO₂ nanocomposites provide an exceptionally efficient, stable, and sustainable platform for the photocatalytic remediation of recalcitrant emerging antibiotics in aquatic environments.
Future research will concentrate on continuous-flow pilot photoreactor design, immobilization on porous ceramic membranes, and pilot-scale trials with authentic municipal wastewater effluents.
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